1. Field of the Invention
This invention relates to a photocatalytic colored member that gives visual coloring effects by achieving the desired optical reflection/transmission characteristics through the interference or diffraction of light, and also has a photocatalytic effect which has the effect of decomposing organic material adsorbed to the surface of raw material with moisture or by irradiation of ultraviolet light the addition of moisture or irradiation with ultraviolet light.
2. Description of the Prior Art
Titanium dioxide or other so-called photocatalytic materials are known to exhibit such effects as the decomposition of organic matter or the electrolysis of water when irradiated with ultraviolet light, and various consumer products that exploit this property have been developed. Among these, exterior wall covering material using photocatalytic material is able to photocatalyze adhering contaminants and algae and the like so that it can be rinsed off naturally with rainwater, and thus the costs of washing exterior walls can be reduced. In addition, if photocatalytic material is used as a component of interior walls, it is known to decompose gases and adhered substances which are hazardous for humans. In order for photocatalytic materials to be further popularized as interior and exterior wall covering materials (hereinafter referred to as “members”), it is necessary to establish techniques for coloring the members with the desired color.
There are several conceivable techniques for obtaining members of the desired color with the photocatalytic effect maintained. For example, there is a technique of dispersing a powder of photocatalytic material within a paint of a certain color and painting the member. With this method, there is a problem in that the photocatalytic effect ocurrs only at that portion of the photocatalytic material exposed upon the surface, so the surface area of contact with contaminants cannot become greater than the surface area of one layer of photocatalytic powder, and thus a high photolytic effect cannot be expected (Problem 1).
Moreover, the photolytic effect due to photocatalysis extends not only to the contaminants but also to the paint itself. Therefore, those portions of the substances making up the paint that are in contact with the photocatalytic material are subject to photolysis so not only will the color deteriorate but the photocatalytic material will fall off, so the coloring and photolytic performance of the paint film cannot be maintained over the long term (Problem 2). This phenomenon is a well-known phenomenon known as the chalking of paint.
Since this photolytic effect does not occur unless the photocatalytic material comes into direct contact with the substance to be decomposed, techniques have been proposed whereby silicon dioxide or another inorganic substance that is not subject to the effects of the photocatalytic effect is mixed together with the pigment and formed, or the powder of photocatalytic material is partially coated with material that is not subject to the photocatalytic effect and then dispersed within the paint or the like. However, with such techniques, the surface area of contact between the photocatalytic material and the substance to be removed is decreased, so the photolytic performance is further decreased and thus a high photolytic effect cannot be expected as in the case of Problem 1.
There is also a method of adhering powdered photocatalytic material to a resin sheet or other carrier material. In order to obtain coloring by this method, the resin sheet will be colored, so there are limits to coloring, and decomposition and deterioration of the carrier material occurs due to the photolytic effect. While relatively durable carrier materials made of Teflon have also been developed, the heat-resistance temperature is the decomposition temperature of Teflon or roughly 300° C., so this cannot be used in materials for heat-resistant applications (Problem 3).
Conceivable methods of obtaining coloring without using dye or pigment include methods that utilize the interference or diffraction of light. For example, there is a method wherein a sub-micron titanium oxide thin film is formed upon the surface of a sheet of metallic titanium by means of anodization and then coloring is obtained by the effect of interference of light between the titanium plate and the surface of titanium oxide. While semi-permanent coloring can be obtained with this method, it has problems in that:                (1) oxidation is performed by passing a current through the metallic titanium plate, so it is not applicable to transparent glass substrates or other diverse types of substrates,        (2) a metallic titanium surface with a high smoothness is required for the substrate, and        (3) the structure of the titanium oxide formed by anodization is said to be amorphous, not the anatase structure said to have a high photolytic effect, and thus a large photolytic effect cannot be expected (Problem 4).        
Moreover, in order to obtain coloring in the visible range, a sub-micron film thickness of titanium oxide is required, and in order to obtain the desired color uniformly, even finer film-thickness control down to {fraction (1/10)} of that is required, among other problems (Problem 5).
In addition, another conceivable coloring method that does not use dye or pigment makes use of diffraction gratings. A diffraction grating is an optical component formed by creating a fixed periodic structure upon a substrate, known to have the property of reflecting light of a specific wavelength depending on the period at a certain angle. However, an ordinary diffraction grating has a wide range of wavelengths of reflected light, and the wavelength of the reflected light depends on the angle, so the color changes depending on the viewing angle of the diffraction grating, thus giving a rainbow effect from red to violet, for example, and it is impossible to obtain the desired color over a wide angular range (Problem 6).
In addition, since a diffraction grating has a fine structure on the μm order, dust on its surface or the adhesion of contaminants will spoil its performance. For this reason, unless the surface is covered with a clear material or similar steps are taken, a diffraction grating can be used only in a special clean environment.
Therefore, present invention has as its object to provide a photocatalytic colored member that has a photocatalytic effect higher than that of conventional materials, and can also maintain the desired clear color for the long term.